The proliferation of digital media such as image, video and multimedia is creating a need for a security system which facilitates the identification of the source of the media and/or access control information. The expected introduction of digital video disks (DVD) in the mass market will further exacerbate the problem.
Content providers, i.e. owners of works in digital data form, have a need to embed signals into video/multimedia data, including audio data, which can subsequently be detected by software and/or hardware devices for purposes of authenticating copyright ownership, controlling copying and display and ownership management, this is known as "watermarking."
A preferred method of watermarking or embedding signals in an image or image data is to insert the watermark into n.times.n blocks (subimages or subblocks) of the N.times.N image where n&lt;&lt;N. If the block size is chosen to be 8.times.8, i.e. the same size as that used for MPEG image compression, then it is possible to tightly couple the watermark insertion and extraction procedures to those of the MPEG compression and decompression algorithms. Considerable computational savings can then be achieved since the most expensive computations relate to the calculation of the DCT and its inverse and these steps are already computed as part of the compression and decompression algorithm. The incremental cost of watermarking is then very small typically less than five percent of the computational requirements associated with MPEG.
A review of watermarking is found in an article by Cox and Miller, entitled "A review of watermarking and the importance of perceptual modeling", in Proc. of EI'97, vol. 30-16, Feb. 9-14, 1997.
To allow for computationally efficient detection of the watermark in both the spatial and DCT domains, a watermark is inserted into sums of groups of 8.times.8 blocks in the DCT domain. The advantage of this approach is that, if the image is only available in the spatial domain, then the summation can also be performed in the spatial domain to compute a small number of 8.times.8 blocks and only these blocks must then be transformed into the DCT domain. This is because the sum of the DCT blocks is equal to the DCT of the sum of the spatial blocks. Since the computational cost of detecting watermarks is now dominated by the cost of summation, the cost of detecting in the DCT and spatial domains is approximately the same.
In U.S. patent application Ser. No. 08/928,576, filed Sep. 12, 1997, entitled "Counteracting Geometric Distortion for DCT Based Watermarking" having inventors I. J. Cox and M. J. Miller and assigned to the same assignee as the present invention, which is incorporated herein by reference, an image is divided into M disjoint sets of n.times.n blocks. The detection process comprises the steps of step (1) averaging all of the n.times.n subblocks in each of the disjoint groups to a single n.times.n block such that each frame is reduced to M n.times.n subblocks, referred to as an accumulator array. The averaging may be performed in either the pixel domain or the DCT domain. Step (2) the M n.times.n subblocks are further reduced to a one-dimensional vector of length M which is referred to as the extracted watermark. Step (3) a standard statistical test using correlation coefficients is used to compare the extracted watermark to a known set of possible watermarks to determine which known watermark, if any, is present in the image. Step (4) in the referenced patent application, is to repeat steps (2) and (3) after modification of the accumulator array to compensate for geometric distortion.